Data that is stored in a storage system includes pieces of data having high access frequency, pieces of data having low access frequency, pieces of data that are rarely accessed, and the like. Storage devices that are incorporated into a storage system may be a combination of storage devices with relatively high access performance such as solid state drives (SSDs) and storage devices with relatively low access performance such as hard disk drives (HDDs).
In general, the storage devices with high access performance have a higher cost per data storage capacity than the storage devices with low access performance. The storage system may improve overall access performance at low cost while having a large capacity without using high-performance storage devices for all storage devices included therein, by determining a storage device to serve as a storage destination based on data characteristics, such as access frequency.
Allocation of data to a storage device to serve as a storage destination depending on data characteristics, such as that described above, is known as a hierarchical control method in the storage system. A hierarchical control method is also known that monitors changes in data characteristics and changes the storage device to serve as the storage destination depending on the changes in data characteristics.
Related techniques are disclosed in, for example, Japanese Laid-open Patent Publication No. 2000-187608 and Japanese Laid-open Patent Publication No. 7-306757.
A technique is known in which a virtual volume is configured by receiving allocation of storage area in SUB-logical unit number (LUN) units from the storage device. In this instance, the storage system is capable of performing hierarchical control in which the storage device to serve as the storage destination is changed in SUB-LUN units.
As a result, even when evaluation of data characteristics per volume unit is difficult, the storage system may evaluate the data characteristics in the smaller SUB-LUN units, and allocate storage areas to the storage device in SUB-LUN units.
However, even when allocation is performed in SUB-LUN units to the storage device, changes to the allocation of storage area may be desired to be performed during a time period in which user access load is light, to suppress effect on user access. Therefore, the changes to the allocation of storage area are not able to track load fluctuations on volumes in a manner close to real-time. The effects of hierarchical control may not be sufficiently realized.